So in this case, the core framework being exposed to javascript is evidence of the expectation of external users, thus it will likely not be a moving target. If you're modifying some software, you will greatly prefer programming to an API than not.
In this case, going with Chrome would mean very invade changes to enable exposing things the upstream specifically wants to keep private in their codebase. Firefox instead requires only well-delineated, minimally invasive changes because there is a nice clean boundary between UI code and engine code (which incidentally matches c++/js boundary, the languages themselves don’t matter much), which means their changes can be isolated to the UI layer, which in turn means a much cleaner path to merging upstream changes